A patentability search directed to this invention has identified the following references as of possible interest: U.S. Pat. Nos.
2,711,419 Milbourne et al PA1 2,759,806 Pettyjohn et al PA1 3,424,808 Brewer et al PA1 3,511,624 Humphries et al PA1 3,759,679 Franz et al PA1 3,866,353 Krumm et al PA1 3,870,481 Hegarty PA1 3,975,169 Gent PA1 3,990,867 Baron et al PA1 4,010,008 Jo PA1 4,017,274 Galstaun PA1 4,065,514 Bartley et al PA1 4,115,075 McNamee et al PA1 4,209,305 Conway et al
Additionally, U.S. Pat. No. 3,732,085--Carr et al appears to be of interest.
Carr discloses a process for producing synthetic natural gas from a sulfur-containing crude oil wherein a portion of the 1040+.degree. F. residual is partially oxidized to produce hydrogen and this hydrogen is used to hydrocrack and desulfurize a 375.degree.-1040.degree. F. heavy oil cut to produce naphtha. This naphtha is combined with the naphtha cut from the crude distillation column and the overhead from a vis breaker for conversion to synthetic natural gas in a sequence of units, as illustrated in FIG. 2, including a steam reformer 142, a hydrogasifier 146, and a methanator 150.
In the process of the Conway patent, crude oil is also fractionated into a heavy end which is partially oxidized to produce hydrogen for a hydrogenation process, such as a gas recycle hydrogenation unit (20 in FIG. 1) while a lower boiling point fraction of the crude is gasified such as by a "catalytic-rich gas steam reforming process" (50 in FIG. 1). An ethane stream 45 fractionated from the gasifier product may be combined in this gasification unit 50. It is not known if the process necessarily includes a methanation stage. The cut-off between the middle fraction, which is reformed, and the top fraction, which is hydrogenated in this process, is about 200.degree. C. or 392.degree. F.
U.S. Pat. No. 3,975,169--Gent discloses a process wherein thermal efficiency is enhanced by the combination of sequential steam reforming and methanation to produce synthetic natural gas. There appears to be no suggestion in this patent of the combination of these processes so that the exothermic and endothermic reactions would occur together.
Brewer et al does disclose a catalyst for combined exothermic and endothermic reactions. The reactions involved in Brewer et al, however, are methanation and dehydrogenation (to produce olefins).
Sequential steam reforming and methanation of a hydrocarbon stream is seen in the disclosures of the Milbourne et al, Baron et al, and Jo patents. The further sequential inclusion of hydrogasification in such a process is also seen in the McNamee et al patent. In the latter patent, numerous heat exchangers are included for enhancing the thermal efficiency of the process and reference is made to the exothermic nature of certain of the reactions and endothermic nature of other reactions.
Galstaun is concerned with enhancement of a methanation reaction only, as is Humphries et al which discloses a two-stage methanation process, while Krumm et al relates primarily to a two-stage reforming process.
Franz et al and Bartley et al relate primarily to specific catalysts for hydrocarbon conversion to synthetic natural gas and Pettyjohn et al is of interest, only because of its teaching of steam conversion of a portion of a natural gas liquid feedstock to produce hydrogen for hydrocracking treatment of the remainder of the feedstock.
U.S. Pat. No. 3,870,481--Hegarty, of common assignment with the present invention, is of interest for its teaching of an improved process for the hydrogasification of higher boiling point crude oil fractions by first vaporizing the feedstock in combination with hydrogen and subsequently gasifying the vaporized steam in a gas recycle hydrogenation process.
The background portion of this patent includes an extensive recitation of other processes for making synthetic natural gas, including steam reforming and gas recycle hydrogenation.
Apart from the prior art discussed above, the inventor here is also aware of another process wherein synthetic natural gas is produced in a more thermally efficient manner by a combined steam reforming and methanation process, wherein the exothermic and endothermic reactions are conducted together. However, it is believed that the integration of this process with a hydrogasification process has not been known or suggested by others prior to the present invention.
More specifically, while it has been known that hydrogasification processes, particularly such processes adapted to the gasification of relatively high boiling point hydrocarbon feedstock streams have been known to be a relatively efficient route to produce synthetic natural gas, the efficiency of such a process has been impaired to some extent by the inefficiency of the process in handling materials with a boiling point above 750.degree. F. and in efficiently using the aromatic-rich by-products of the process. One of the difficulties is that if all of the 750.degree. F.+ fraction is partially oxidized (to produce synthesis gas, part of which is then convertible to methane), more hydrogen than can be used in the hydrogasification process is produced.
With all of these factors in view, there remains a need for a more efficient process for converting high boiling point (500.degree. +F.) feedstocks, such as topped crude oil, into synthetic natural gas. More particularly, there remains a need for enhancing the efficiency of gas recycle hydrogenation processes for converting such feedstock to synthetic natural gas.
The general object of the present invention is to provide such a more efficient synthetic natural gas producing process.